1. Field of the Invention
The present invention relates to a solar cell formed on a semiconductor substrate and a method of producing the same, and particularly to improvements in a solar cell formed on a thin semiconductor substrate liable to fracture and in a method of producing the same.
2. Description of the Background Art
A semiconductor silicon substrate for a solar cell is cut out from a monocrystalline or polycrystalline semiconductor ingot to a thickness of about 300 to 350 μm, using an inner-edge slicer, an outer-edge slicer, a wire saw, or the like. In producing the solar cell, a pn junction for causing photoelectric conversion is formed in the silicon substrate, and then an anti-reflection coating for increasing light entrance into the substrate, an electrode for efficiently extracting photoelectric conversion current, and the like are formed.
In FIGS. 25A to 25C, an example of a conventional solar cell is shown. FIG. 25A shows a front surface provided as a light-receiving face of the cell, FIG. 25B shows a back surface of the same, and FIG. 25C shows a cross-section taken along a line 25X-25X in FIG. 25A. When producing such a conventional solar cell 1, any damaged layer caused by slicing off a silicon wafer 1a is initially removed from the wafer by alkaline etching. An n-type impurity is then applied on a surface of silicon substrate 1a, and a pn junction is formed near the light-receiving face side by thermal diffusion. On the light-receiving face side of substrate 1a, an anti-reflection coating (not shown) is formed with plasma CVD (chemical vapor deposition), and on the back surface, a backside collector electrode 1c is formed by printing and baking aluminum paste with a screen process. Further, by printing and baking silver paste with the screen process, a comb-like collector electrode 1b is formed on the light-receiving face side of substrate 1a, and a connection electrode 1d is formed on the back surface. The surfaces of electrodes 1b, 1d composed of baked silver are coated with solder layers 1e by dip soldering.
Further, interconnectors made of conductive metal bands are connected between solar cells, to fabricate a solar battery module including several tens of solar cells. In the solar battery module, a glass plate is joined with transparent resin to the front side of the several tens of solar cells arranged in an array, and then an insulating film, a moisture-proof film and the like are provided on the backside. Terminals for externally extracting electric current are drawn from both ends of a solar cell circuit.
In order to reduce cost for such a solar battery module, it is necessary to make thinner semiconductor silicon substrates which account for a large part of the cost, so as to reduce usage of expensive silicon material. If silicon substrates are made thinner, however, fracture of solar cells will increase in production process of the solar cells and modules, resulting in lower production yield. That is, this will not necessarily lead to cost reduction. In addition, if the solar cells are made thinner, the fracture of the same will be liable to occur, for example, due to temperature cycles experienced during use of the completed solar battery modules, and hence the quality and reliability of the modules may be lowered. Therefore, the silicon substrates in the conventional mass-produced solar cells generally have a thickness of about 300 to 350 μm.
In other words, if silicon wafers cut out from a silicon ingot are made thinner, invisibly small cracks, flaws or chips will be liable to occur in edge portions of the wafers in production processes of the silicon substrates, the solar cells and the solar battery modules. Originating from those defects, larger cracks or fractures are likely to be developed. Fractures in the silicon substrates during the manufacturing process of the modules cause loss of substrate material, replacement of the cracked substrates and need for additional work for removing cracked pieces from manufacturing apparatus. As a result, the cost of the solar cells and the modules will be increased. In particular, if a solar cell substrate (a wafer) fractures during production of the solar battery module, the entire module will be defective and resulting in larger loss. Even if the solar battery module is flawless immediately after its completion, fractures may likely be caused later in the cells, for example, by temperature cycles during use of the module in the case that the solar cell substrates are thin.